The National Hurricane Center, uses the term “tropical cyclone” or “TC” as a generic term for a non-frontal synoptic-scale (i.e., on the order of 1000 km) low-pressure system over tropical or sub-tropical waters, where the system exhibits organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation.
Intense tropical cyclones can wreak havoc on coastal areas, destroying property and threatening life. Even if such storms do not make landfall, tropical cyclones create significant problems for both military and commercial maritime activities, often requiring these activities to make expensive and time-consuming changes to their operations.
Accurate forecasts of the track, intensity and size of tropical cyclones is thus critical for operations planning and for avoiding damages and loss of life.
Together, the U.S. TC Forecast Centers (the Joint Typhoon Warning Center, National Hurricane Center, and Central Pacific Hurricane Center) perform analysis and forecasting of TCs around the clock and for the entire globe for U.S. assets and allies. This analysis and forecasting are performed at a set series of times based on the universal time coordinate (UTC) standard, and are currently performed at UTC 00:00, 06:00, 12:00, and 18:00.
The analysis and forecast process at the U.S. TC Forecast Centers includes a series of steps that define and predict several descriptive parameters for each TC. A large portion of that process is in defining and predicting the TC “wind radii,” i.e., the maximum distance from the TC center that winds of a certain level extend. At the U.S. TC Forecast Centers, the wind radii are defined for 34-knot (kt), 50-kt, and 64-kt thresholds. These levels are chosen because they correspond to gale force (34 kt), damaging (50 kt), and typhoon/hurricane force (64 kt) winds. These wind radii are further broken up into compass-based quadrants (NE, SE, SW, and NW) since each TC has its own frequently non-symmetric structure, causing the wind radii to be different in each of these four quadrants.
Analysis of wind radii is performed using an analytical framework often referred to in the art as the “objective track” or “OBTK,” while the wind radii forecast is performed using an analytical framework often referred to in the art as the “wind radii consensus” or “RVCN.”
These wind radii are critical to the forecast in that they provide a measure of the severity and geographic extent of the TC.
The results of these wind radii calculations and/or forecasts are typically shown in graphical form on plots such as those shown in FIGS. 1A and 1B.
FIG. 1A shows the results of a single exemplary wind radii analysis, and shows an exemplary wind radii plot 101, which includes a plot showing estimated wind radii 101a that contains TC winds 34 kt and above, 101b contains winds 50 kt and above, and 101c contains winds 64 kt and above. Each of these wind radii is further divided into the four geographic quadrants 102a-102d, showing the wind radii in each of those four quadrants. As can be seen in the FIG. 1A, the radii for winds at higher speeds (e.g., 50 kt radius 101a) are smaller than those for lower speed wind radii (e.g., 34 kt radius 101a). This is expected because TCs generally have highest speed winds near the center of the storm, and then the wind speeds generally decay with distance from its center.
FIG. 1B illustrates the results of an exemplary analysis and forecast of an intense TC with winds greater than 64 kt both at in the analysis and throughout a 5-day forecast. Such an analysis/forecast includes both an analysis of wind radii at a single analysis time shown by plot 101, plus the results of a forecast of the wind radii at 12, 24, 36, 48, 72, 96, and 120 hours in the future, shown by plots 102-105. The wind radii plots produced as part of the forecast process will often also include an historical track 110, showing where the TC has been and a forecast track 120, where the forecast wind radii follow a forecast track of the cyclone.
For a forecaster to define all of these wind radii for a single specified time (e.g., perform only a single analysis), the number of individual wind radii estimates required would be the number of wind speed thresholds times the number of wind radii quadrants (i.e., 3 wind speed thresholds×4 wind radii quadrants), or 12 individual wind radius estimates. If a forecaster were to estimate the wind radii for, e.g., 7 times in the future (e.g., the wind radii at 12, 24, 36, 48, 72, 96, and 120 hours in the future), in addition to making an estimate of the wind radii at the current time, the number of wind radius estimates required would become 3×4×8=96 individual wind radius estimates.
Doing each of these calculations individually rapidly becomes untenable, since the time allotted to a forecaster to complete the entire TC forecast, including performing other tasks such as communicating with customers and disseminating appropriate warnings to the affected sites, is generally less than 2 hours. In addition, some forecasters (e.g., the forecasters at the Joint Typhoon Warning Center) are required to generate forecasts—including wind radius forecasts—for more than one TC, which further increases the burden on the forecasters.
Consequently, there is a strong need for algorithms and convenience tools to facilitate and improve the performance of these tasks.